Selecting high-performance ac spindle motors

Technologies are opening new choices for machine tool spindle motors. This article will help you make a better choice

If your spindle application calls for moderate speed regulation, and will operate at one or two defined speeds, then a standard ac motor may be a candidate. Speed regulation would be limited to the slip of the motor, approximately 1.5 to 3% of base speed. Some applications use a one or two-speed motor with or without a changeable-ratio gear box.

However, for a high-performance adjustable- speed operation, a vector controller with an ac spindle motor should be used.

Spindle motor vs. standard motor.

High-peak-torque capability combined with low rotor inertia of a high-performance spindle motor enables higher acceleration rates to base speed than are available with a standard ac motor. Figure 1 shows the acceleration time for a standard TENV ac motor and a high-performance ac spindle motor. Both are driving a tool load with an inertia of 0.25 lb-ft2 and are powered at their continuous rating of 10 hp. The spindle motor, with a rotor that has 24% of the inertia of a standard ac motor, reaches the set speed in 28% of the time required for the standard motor. This shorter acceleration time produces faster orientation capability for tool changing, for example.

Operating ranges

The general laws of physics determine the required torque values to accelerate and decelerate loads powered by spindle motors. (Many of the basic equations for calculating the motor speed and torque requirements are covered in the PTD 1997 Handbook, 12/96, p. A24; also see the note at the end of this article.)

Many machine tool spindle drives require maintaining a constant tool force and a constant surface speed at the cutting tool. This requirement defines constant power by the drive, because as the radius decreases the torque decreases (for a constant cutting force) and the speed should increase to maintain a constant cutting velocity.

Thus, to be economical, most spindle motors should be able to operate in a constant horsepower range, Figure 2. Without this capability, the motor would have to be significantly oversized.

Some typical numbers have been inserted in Figure 2 to illustrate an example. The important aspects are the constant torque region, which extends from zero speed to 1,500 rpm (base speed), and the constant horsepower region which starts at base speed and continues to 5,250 rpm, which is 3.5 times the base speed.

In the region up to base speed, it is important to have a high constant torque available. This is attained with the motor’s electrical winding connected in a wye configuration. This connection is best for large-diameter, rough-cutting operations, which typically require high torque at low speeds.

With a base speed of 2,600 rpm, the constant horsepower range is 2,600 x 3.5 = 9,100 rpm with the delta connection. When connecting the motor in a delta configuration and extending the maximum speed, verify that you are not exceeding the motor’s maximum safe operating speed. The delta connection is best for high-speed finishing cuts, typically made by small-diameter tools.

Speed control

To provide tight speed and torque control, a feedback device, typically an encoder or resolver, must be added to the motor. Then a vector control can be used to power the spindle motor.

With vector control, tighter speed regulation approaching 0.01% of set speed can be attained. Vector controls and high-performance spindle motors, Figure 4, have a constant power range of about 3.5 times base speed and controllable speed ranges from zero speed to about five times base speed.

Smooth stopping

Whenever the load causes the motor to spin faster than the applied frequency, then the motor becomes a generator. That is, if the applied frequency is decreased to, say, 30 Hz while the motor is still running nearly at 60 Hz, the motor will generate energy back to the drive. Then, the drive must do something with that energy. One way is to shunt it through a dynamic braking (DB) resistor, which dissipates the energy as heat.

If the application has high inertial loads or needs controlled fast stopping, then consider a line-regenerative vector control. Such a drive puts the regenerated energy back into the ac power line, thus reducing energy costs. Additionally, since these designs operate near unity power factor, they produce additional energy savings.

A typical vector control with line regeneration capabilities should include an active bridge input stage which meets IEEE Standard 519-1992. This standard, “Recommended Practices and Requirements for Harmonic Control in Electric Power Systems” is a recommendation on limits of harmonic current drawn from, and fed into utility power lines.

Bearings and overhung loads.

If your application is operating at nominal base speeds with direct-coupled loads, ball bearings are adequate. The motor is usually precision balanced for speed operation to 8,000 rpm. For overhung loads, and coupled or belted loads, roller bearings should be considered, as these enable higher radial forces (up to four times) to be applied to the shaft.

Duty cycle

To handle the constant high-power demands, and the heat that comes with it, spindle motors are usually supplied with separately powered constant-velocity fans. These fans provide cooling that is independent of the motor’s operating speed. Thus, the motor can be used at low speeds as well as at higher constant power ranges without reducing the motor’s duty-cycle factor.

Overload conditions require torque levels above the continuous rating of the spindle motor. This condition would normally cause the motor to overheat. However, it is possible to get more torque, more power out, without overheating, by simply observing the duty-cycle curve of the motor.

Figure 5 shows a typical speed-torque curve for a 10-hp spindle motor, along with curves illustrating extra torque that the motor can deliver if the duty cycle is considered. These curves are based on IEC-34 “Type S3 Duty.” For example, the motor can deliver 35 lb-ft (47 Nm) continuously at 1,500 rpm (100% duty cycle curve). However if the application requires extra torque, the motor can provide 75 lb-ft (101 Nm) at 1,500 rpm. The motor can provide this torque without overheating, as long as duty cycle is maintained at 25%. Note that constant power can be delivered out to 8,000 rpm.

The duty cycle, Figure 6, is defined as the percentage of total cycle time that is on time — the period that current is running through the motor windings. Off time is the amount of time at which the motor windings are de-energized — the cool down period.

Once the duty cycle and horsepower/torque are determined for the application, establish the point on the manufacturer’s literature (curves as shown in Figure 5) to determine whether the motor will operate satisfactorily.